From 3d51d8a581ad5f6c58cae7a840043e67405060f7 Mon Sep 17 00:00:00 2001
From: Siegfried Pammer <siegfriedpammer@gmail.com>
Date: Fri, 29 Aug 2025 20:59:21 +0200
Subject: [PATCH] Fix #3547: Fix duplicate key in
 PropertyAndEventBackingFieldLookup

---
 .../PropertyAndEventBackingFieldLookup.cs     | 36 +++++++++++++------
 .../Util/MultiDictionary.cs                   | 11 ++++++
 2 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/ICSharpCode.Decompiler/Metadata/PropertyAndEventBackingFieldLookup.cs b/ICSharpCode.Decompiler/Metadata/PropertyAndEventBackingFieldLookup.cs
index b28eb4264..e0882cdb7 100644
--- a/ICSharpCode.Decompiler/Metadata/PropertyAndEventBackingFieldLookup.cs
+++ b/ICSharpCode.Decompiler/Metadata/PropertyAndEventBackingFieldLookup.cs
@@ -19,6 +19,8 @@
 using System.Collections.Generic;
 using System.Reflection.Metadata;
 
+using ICSharpCode.Decompiler.Util;
+
 namespace ICSharpCode.Decompiler.Metadata
 {
 	class PropertyAndEventBackingFieldLookup
@@ -33,7 +35,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		{
 			this.metadata = metadata;
 
-			var nameToFieldMap = new Dictionary<string, FieldDefinitionHandle>();
+			var nameToFieldMap = new MultiDictionary<string, FieldDefinitionHandle>();
 
 			foreach (var tdh in metadata.TypeDefinitions)
 			{
@@ -51,14 +53,22 @@ namespace ICSharpCode.Decompiler.Metadata
 					var property = metadata.GetPropertyDefinition(pdh);
 					var name = metadata.GetString(property.Name);
 					// default C# property backing field name is "<PropertyName>k__BackingField"
-					if (nameToFieldMap.TryGetValue($"<{name}>k__BackingField", out var fieldHandle))
+					if (nameToFieldMap.TryGetValues($"<{name}>k__BackingField", out var fieldHandles))
 					{
-						propertyLookup[fieldHandle] = pdh;
+						foreach (var fieldHandle in fieldHandles)
+						{
+							propertyLookup[fieldHandle] = pdh;
+						}
 					}
-					else if (nameToFieldMap.TryGetValue($"_{name}", out fieldHandle)
-						&& fieldHandle.IsCompilerGenerated(metadata))
+					else if (nameToFieldMap.TryGetValues($"_{name}", out fieldHandles))
 					{
-						propertyLookup[fieldHandle] = pdh;
+						foreach (var fieldHandle in fieldHandles)
+						{
+							if (fieldHandle.IsCompilerGenerated(metadata))
+							{
+								propertyLookup[fieldHandle] = pdh;
+							}
+						}
 					}
 				}
 
@@ -66,13 +76,19 @@ namespace ICSharpCode.Decompiler.Metadata
 				{
 					var ev = metadata.GetEventDefinition(edh);
 					var name = metadata.GetString(ev.Name);
-					if (nameToFieldMap.TryGetValue(name, out var fieldHandle))
+					if (nameToFieldMap.TryGetValues(name, out var fieldHandles))
 					{
-						eventLookup[fieldHandle] = edh;
+						foreach (var fieldHandle in fieldHandles)
+						{
+							eventLookup[fieldHandle] = edh;
+						}
 					}
-					else if (nameToFieldMap.TryGetValue($"{name}Event", out fieldHandle))
+					else if (nameToFieldMap.TryGetValues($"{name}Event", out fieldHandles))
 					{
-						eventLookup[fieldHandle] = edh;
+						foreach (var fieldHandle in fieldHandles)
+						{
+							eventLookup[fieldHandle] = edh;
+						}
 					}
 				}
 
diff --git a/ICSharpCode.Decompiler/Util/MultiDictionary.cs b/ICSharpCode.Decompiler/Util/MultiDictionary.cs
index 9b113870e..7dd882c30 100644
--- a/ICSharpCode.Decompiler/Util/MultiDictionary.cs
+++ b/ICSharpCode.Decompiler/Util/MultiDictionary.cs
@@ -86,6 +86,17 @@ namespace ICSharpCode.Decompiler.Util
 			}
 		}
 
+		public bool TryGetValues(TKey key, out IReadOnlyList<TValue> values)
+		{
+			values = EmptyList<TValue>.Instance;
+			if (dict.TryGetValue(key, out var list))
+			{
+				values = list;
+				return true;
+			}
+			return false;
+		}
+
 		/// <summary>
 		/// Returns the number of different keys.
 		/// </summary>